1. Field of the Invention
The present invention relates to an image forming apparatus for transferring a toner image onto a recording material by using a belt member for directly or indirectly conveying the toner image. More particularly, the invention relates to such a technique that a disturbance of a toner image which is conveyed is prevented by controlling a supporting member for supporting a belt member.
2. Description of the Related Art
An image forming apparatus in which toner images of separating colors formed on photosensitive drums are overlaid and transferred onto an intermediate transfer belt and a full-color image is formed has been put into practical use. An image forming apparatus in which toner images of different separating colors are overlaid and transferred onto a recording material which has been adsorbed onto a recording material conveying belt and conveyed thereby and a full-color image is formed has also been put into practical use. A full-color image forming apparatus in which a photosensitive drum to form a toner image of black is arranged on the most downstream side, and in the case of forming a monochrome image, only the photosensitive drum to form the black toner image is used has also been put into practical use.
Image forming apparatuses in each of which a belt member is separated from image bearing members for forming toner images of separating colors other than black and a monochrome image is formed by a photosensitive drum on the most downstream side have been disclosed in Japanese Patent Application Laid-Open Nos. 2001-249519, 2004-117426, and 2005-62642. The belt member is separated and the photosensitive drums and a charging apparatus which are not concerned with the image forming are stopped, thereby preventing an unnecessary mechanical abrasion and saving electric power consumption.
A supporting roller for supporting an inside surface of the belt member and keeping a contacting position of the belt member on the photosensitive drum on the most downstream side constant is arranged between the two photosensitive drums located on the most downstream side. When the monochrome image is formed, the belt member is bent and supported by the supporting roller on the downstream side separated from a plurality of photosensitive drums.
In the image forming apparatuses disclosed in the above-identified patent documents, it has been found that when the color image is formed by circulating the intermediate transfer belt, a disturbance occurs in the toner image on the intermediate transfer belt. As will be described hereinafter, it has been confirmed that in the case of circulating the intermediate transfer belt, when the belt passes through the supporting roller, an uneven charge occurs in the intermediate transfer belt and the toner image on the intermediate transfer belt is scattered. Specifically, a toner scattering phenomenon of a thin line image or a lateral striped flutter in a halftone image occurs.